An aluminum solid electrolytic capacitor or a tantalum solid electrolytic capacitor is known as a capacitor having high capacitance and low ESR (equivalent series resistance) to be used for various electronics.
A solid electrolytic capacitor is fabricated by encapsulating a solid electrolytic capacitor element comprising one electrode (anode body) made from an aluminum foil having fine pores on the surface layer or a sintered body of tantalum powder or niobium powder having fine pores inside; a dielectric layer formed on the surface layer of the electrode; the other electrode (cathode, which is usually a semiconductor layer) provided on the dielectric layer; and an electrode layer laminated on the other electrode. In cases of anode bodies of the same volume, the smaller the fine pores there are and the more fine pores there are in the anode body, the surface area inside the anode body becomes larger, resulting in the higher capacitance produced from the anode body.
The dielectric layer is formed by an electrochemical method so-called chemical formation. Examples include a method of forming the dielectric layer by dipping a conductor layer in an electrolytic solution in which mineral acid such as phosphoric acid and sulfuric acid or salt thereof, or organic acid such as acetic acid, adipic acid, benzoic acid or salt thereof is dissolved; and by applying a predetermined voltage between the conductor layer as an anode and the cathode separately provided in the electrolytic solution.
JP-A-S50-100570 publication (Patent Document 1; U.S. Pat. No. 3,864,219) discloses the chemical formation in an electrolytic solution using quaternary ammonium salts. JP-A-S50-102861 publication (Patent Document 2) discloses the chemical formation using an electrolytic solution of boric acid and the like.
A conductive organic compound or inorganic compound can be used as a cathode, but a conductive polymer is preferably used in view of the heat resistance and low ESR property of the capacitor produced thereof. The conductive polymer means a polymer having conductivity as high as 10−2 to 103 S·cm−1 which exerts high conductivity by adding an electron-donating compound so-called a dopant to a polymer having a conjugated double bond (which is generally an insulating polymer or a polymer having an extremely low conductivity). One of the specific examples of forming a conductive polymer layer as a cathode is a method of polymerizing the monomer which can be turned into a conductive polymer in the above-mentioned fine pores of the anode body in the presence of dopant by supplying thereto an appropriate oxidizing agent or electrons. The dopant taken in when the monomer is polymerized produces a strong interaction with the polymer having a conjugated double bond, and thereby a conductive polymer is obtained.
In production methods of solid electrolytic capacitors, the operation of impregnating an anode body comprising valve-acting metal on which a dielectric film is formed with a solution for forming a cathode such as manganese nitrate solution, conductive monomer solution and conductive polymer dispersion liquid is known technique.
However, in the method the cathode-forming solution penetrates inside the anode body through capillarity using that the anode body made of valve-acting metal is a porous body, and the method cannot be directly applied in the cases where the solution is a low-temperature or high-concentration solution having high viscosity. Also, in the conventional method for impregnating the anode body with manganese using a manganese nitrate solution, it has been a usual method to raise the concentration of the impregnating solution with each dipping and it has been necessary to use a diluted solution in the initial dipping, and the number of times and the period cannot have been reduced.
For example, JP-A-2001-338845 publication (Patent Document 3) and JP-A-2001-155966 publication (Patent Document 4) show a method for forming a conductive polymer compound as a cathode material on an aluminum formation foil. However, the penetration depth of the cathode-formation solution to go deep into the foil becomes physically shallower when an aluminum foil is used compared to the case where a powder sintered body is used. Therefore the method to be used for an aluminum formation foil is not suitable for a powder sintered body having deep fine pores in which the penetration depth for the cathode-forming solution to go should be greater.
Also, a method for conducting the impregnating step under pressure has been suggested.
JP-A-2001-110681 publication (Patent Document 5) suggests a method for forcing the cathode-forming solution to penetrate into an aluminum formation foil by applying pressure. However, the penetration depth is also shallow as in the above-mentioned method. Also, considerable amount of the compressed atmosphere remains in the depth of pores. Since the anode body comprising powder sintered body having deep pores has difficulty in deaerating the atmosphere such as air as in being impregnated with the cathode-forming solution, the anode body is vulnerable to damages of the cathode material and a dielectric body due to the expansion of gas inside the sintered body when subjected to the heat by drying and the heat distortion after the recovery from the thermal injury by the application of voltage.
A method of impregnating a powder sintered body has also been suggested.
For example, JP-A-2005-109252 publication (Patent Document 6) describes a method for forming a polymer compound by impregnating an anode body with an organic compound solution and further impregnating the anode body with a conductive polymer dispersion liquid. In this case, an anode body wherein the tantalum powder capacitance is 40,000 μFV/g is used; and the apertures on the surface of the anode body have relatively large diameter in general and the average diameter of fine pores exceeds 0.5 μm. Therefore, the width of the ingress channel for the cathode-forming agent is large and there is no hindrance in forming a cathode. However, when fine tantalum powder: e.g. powder of having capacitance of 70,000 μFV/g is used, the apertures on the surface of the anode body become smaller and the average pore diameter becomes smaller than 0.5 μm, which makes the ingress channel relatively narrower. In the anode body having such a form, the cathode-forming solution cannot sufficiently penetrate inside the anode body.
Furthermore, with respect to the powder sintered body, a method for conducting the impregnating step under reduced pressure has been suggested.
Specific examples include JP-A-2006-310365 publication (Patent Document 7). However, when an anode body is impregnated with a cathode-forming agent using the method and the average diameter of fine pores becomes less than 0.5 μm, the cathode-forming agent penetrates into the anode body by impregnation under reduced pressure, but the cathode becomes readily formed in the course of volatilization of a solvent during thermal decomposition and drying and, in particular, a cathode is to be selectively formed at the apertures which are the point where the evaporation occurs: i.e. on the surface of the anode body. When the cathode is formed on the surface of the anode body, the fine pores in the vicinity become closed and make it difficult for the cathode-forming agent to penetrate inside the anode body.
This tendency can be commonly seen any anode body in principle, but is particularly pronounced when the average diameter of fine pores in the anode body composed of valve-acting metal is 0.5 μm or less. When the valve-acting metal of the anode body is tantalum powder having capacitance of 70,000 μFV/g or more or niobium powder having capacitance of 130,000 μFV/g or more, it is observed that fine pores on the surface of the anode body are closed. Particularly, the penetration depth of the impregnation solution becomes greater when one side of the sintered body exceeds 1 mm, and the cathode formation inside the anode body becomes incomplete when the closed pores on the surface are observed. In such conditions, a channel for newly supplying a cathode-forming agent is cut off and therefore the solid electrolytic capacitor thereof cannot deliver high performance.
It might be possible to make the cathode-forming solution diluted to prevent the fine pores near the surface of the anode body from being closed at the time of forming a cathode. However, the number of times for impregnation must be increased in this case and therefore the method cannot be considered an efficient method. Also, the prolonged operation time increases the chance of damaging the capacitor element due to the heat at the time of drying.    [Patent Document 1] JP-A-S50-100570 publication    [Patent Document 2] JP-A-S50-102861 publication    [Patent Document 3] JP-A-2001-338845 publication    [Patent Document 4] JP-A-2001-155966 publication    [Patent Document 5] JP-A-2001-110681 publication    [Patent Document 6] JP-A-2005-109252 publication    [Patent Document 7] JP-A-2006-310365 publication